As part of natural gas processing and hydro-treatment of oil fractions, a large amount of hydrogen sulfide (H2S) is produced. The H2S is toxic and therefore is converted to elemental sulfur (S), which is a more practical and safer state for handling and transportation. With more stringent fuel regulations and increasing environmental concerns, together with the need to process sourer crude oils and natural gases, sulfur recovery has become one of the leading issues in emission reduction. Elemental sulfur is the ultimate state of recovery of the sulfur species.
The conversion of H2S into elemental sulfur is performed in a sulfur recovery unit (SRU). The level of sulfur recovery is increasingly important as the need to minimize the amount of sulfur compounds released to atmosphere from the recovery unit needs to be reduced in order to meet the mandated legal limits. The most common process used in the world, for this conversion, is known as the modified Claus treatment process or alternately the Claus unit or modified Claus unit.
The modified Claus treatment process is a combination of thermal and catalytic processes that are used for converting gaseous H2S into elemental sulfur.
Claus unit feed gases have a wide range of compositions. Most of the feed gases originate from absorption processes using various solvents (amine, physical or hybrid solvents) to extract hydrogen sulfide from the by-product gases of petroleum refining, natural gas processing, and also tar sands, coal gasification and other industries. The other gas plants or refinery source of H2S is the sour water stripper unit.
The first process is a thermal process (i.e., in the absence of catalyst) in a reaction furnace. The feed gas to the Claus unit is burned in the reaction furnace using sufficient combustion air, or oxygen enriched air to burn a stoichiometric one-third of the contained H2S. The reaction furnace pressure is maintained at about 1.5 bars (35-70 KPa above atmospheric pressure) and the temperature is maintained at about 900-1,250° C. in a “no-preheat” operation case. The H2S from the feed gas is thermally converted into elemental sulfur, along with sulfur dioxide (SO2). Sulfur yield is around 65%-72% depending on the operation mode of the SRU. Increasing the elemental sulfur yield in the reaction furnace and subsequently the condenser is advantageous as it reduces the later load on the catalytic reactors. The reaction furnace operation is designed to maximize sulfur recovery in consideration of the feed composition, by adjusting air/oxygen feed, reaction temperature, pressure, and residence time. In addition, the reaction furnace can destroy contaminants, such as hydrocarbons, that are present in the feed gas stream. Such contaminants pose problems for the catalytic reactors through the development of carbon-sulfur compounds that can lead to plugging or deactivation of the catalyst beds.
The hot reaction product gas from the reaction furnace, containing sulfur vapor, is used to produce high pressure steam in a waste heat boiler, which also results in cooling the gas. The product gas is then further cooled and condensed in a heat exchanger, while producing additional low pressure steam. The condensed liquid sulfur is separated from the remaining unreacted gas in the outlet end of the condenser and sent to a sulfur pit or other collection area.
The separated gas then enters the catalytic process of the Claus unit. The catalytic process contains between two and three catalytic reactors. Following the sulfur condenser, the separated gas is reheated and enters the first catalytic reactor, which is maintained at an average temperature of about 305° C. In the first catalytic reaction about 20% of the H2S in the feed gas is converted into elemental sulfur through a reaction with the SO2. The temperature is limited by the exit temperature to avoid catalytic bed damages and thermodynamic considerations. The outlet product gas from the first catalytic reactor is cooled, in a second condenser, which can also produce steam. Again, the condensed liquid sulfur is separated from the remaining unreacted gas in the outlet end of the second condenser and sent to sulfur storage. The separated gas from the second condenser is sent to another re-heater and the sequence of gas reheat, catalytic reaction, condensation and separation of liquid sulfur from unreacted gas is repeated for the second and third catalytic reactors at successively lower reactor temperatures. About 5% and 3% of the H2S in the feed gas are converted into elemental sulfur respectively in the second reactor and third reactors.
Finally, the gas stream is released to atmosphere via a stack after passing through an incinerator which oxidizes any remaining sulfur species into SO2. In addition, the flue gas compounds include water, nitrogen, oxygen, sulfide dioxide and eventually carbon dioxide. The eventual presence of carbon dioxide results from the acid gas composition (CO2 and H2S are recovered from natural gas during a sweetening process, such as an amine process). Incinerator temperature and gas temperature in the refractory lined stack are high enough (far above gas dew point) to avoid corrosion and help with quick SO2 dissemination in the surrounding air. Moreover, the stack is designed to make sure SO2 concentration at ground level is below the local regulatory limit.
For a well-designed and well-operated Claus sulfur recovery plant having three catalytic reactors, an overall sulfur conversion of 96-98% can be achieved depending on the feed gas composition. To achieve higher conversion, a tail gas treatment unit must be added to further process the exhaust gas upstream of or as an alternative to an incinerator. Tail gas treatment units are polishing units. Currently available tail gas treatment units can be effective at achieving up to 99.2% recovery, but can add significant capital cost to the Claus treatment unit, often on the same order of magnitude as the Claus unit itself.
Several types of tail gas treatment technologies have been developed and include the Scot® process, Highsulf™, BSR/MDEA™, Sultimate™, Bechtel TGTU, and Technip TGTU. The choice of tail gas treatment unit installed depends on the conversion targeted as cost is directly linked to the required conversion level. While the Scot process can reach 99.9% sulfur recovery, the added cost and unit complexity makes this process unfeasible when the Claus feed is not highly concentrated with hydrogen sulfide, e.g., unless greater than 55%. In addition to increase operating and capital costs, these technologies can require significant physical footprint for the various process vessels, columns, pumps, and storage vessels necessary for operation.
Additionally, processes can be added as an alternative to tail gas treatment units to target SO2 for removal. There are many techniques that have been developed to process exhaust gas in order to reduce sulfur oxide emissions from combusted gas streams. The techniques are generally divided into regenerative processes and non-regenerative processes and can be further divided into wet processes and dry processes.
Non-regenerative processes include a variety of wet-scrubbing processes, such as a limestone-gypsum process and are the leading technologies when high efficiency SO2 removal is targeted at relatively low fcost. In a limestone-gypsum process, flue gas enters an absorber tower and bubbles through a spray of limestone and water, where the SO2 reacts with the lime to create calcium sulfite, which reacts with oxygen to produce gypsum, which can then be disposed. The unreacted gases then exit the top of the tower. The spray tower predominates in the wet desulfurization systems and technologies.
For regenerative processes, the sorbent is reused after thermal or chemical treatment to produce concentrated SO2, which is usually converted to elemental sulfur. These are complex processes requiring high capital outlays and include the magnesium oxide process and Wellman-Lord process. On the dry process side, regenerative processes include the use of activated carbon.
More recently, regenerative processes utilize solvent technologies. Examples of such technologies include: LAB-SORB™, CANSOLV®, ClausMaster™, and Clintox®.
In most cases, flue gas is not saturated. However, before acid gases such as SO2 can be removed, the gas stream must be adiabatically saturated or “quenched.” Most scrubbers will have a section where liquid (typically water or the scrubbing reagent itself) is contacted with the incoming flue gas to adiabatically saturate, or “quench,” the gas stream.
The LAB-SORB™ process utilizes an inorganic regenerable scrubbing reagent to react with SO2. The reagent, rich in SO2 from the scrubber, is processed in a regeneration unit to strip off the captured SO2, producing fresh reagent for scrubbing. The SO2 removed from the reagent is discharged as concentrated/pure SO2 (90+%) and can be sent to the front end of a Claus plant (or sulfuric acid plant) for recovery. Solids are removed from the flue gas in a pre-scrubbing section and de-watered in a system similar to what is used in the purge treatment unit of caustic soda based FCCU scrubbing system. Caustic soda (NaOH) and phosphoric acid (H3PO4) are used for the buffer and small additions are required to make up for small buffer loses. Low pressure steam is used for buffer regeneration in single or double effects evaporation loop. The LAB-SORB′ process produces a minimum amount of waste for disposal, while recovered SO2 can be converted to saleable products such as elemental sulfur, sulfuric acid or liquid SO2. The LAB-SORB′ system can be adapted to many processes, including fossil fuel fired boilers, Claus Tail Gas Treatment, FCCU, Non Ferrous Smelters, Sulfuric Acid Plants, and other SO2 emitting facilities.
The CANSOLV® system is similar to the amine treatment process for removal of H2S and CO2 from refinery streams and natural gas. The gas is contacted counter currently in the absorption tower, where the CANSOLV® solvent absorbs the sulfur dioxide, reducing the effluent gas down to the design SO2 concentration. The rich amine accumulates in the absorption sump. A constant stream of the CANSOLV® solvent (based on a sterically hindered diamine) is withdrawn from the absorption sump to be regenerated in the stripping tower. Once regenerated, the solvent is recirculated to the absorption tower to pick up additional SO2. Emissions as low as 10 ppmV can be achieved. The main part of the CANSOLV® process consists of a structured packing absorption tower and a regeneration tower, also containing structured packing, equipped with a reboiler and an overhead condenser. Associated peripheral equipment consists of process pumps, heat exchangers, and a process particulate filter. The unit also includes an electro dialysis solvent purification unit. Materials of construction are adjusted to handle the lower pH values resulting from the higher acidity of SO2 compared to H2S and CO2. More specifically, stronger acids such as sulfuric and hydrochloric are not released in the regeneration column, ensuring that the product SO2 is of high purity.
In the CLAUSMASTER® process hot SO2 gas is cooled by a DynaWave® wet scrubber and gas cooling tower. SO2 removal occurs only after the SO2 gas has been quenched. This is accomplished in two steps: The acid gases are absorbed into the scrubbing liquid. Once absorbed, the acid gases react with the reagent, forming reaction by-products, which then must be removed from the clean gas. After passing through the proprietary SO2 physical absorbent, clean gas exits the stack and the SO2 is stripped from the SO2 loaded absorbent in the stripping tower. Concentrated SO2 is recycled back to the Claus sulfur recovery plant. The recycled SO2 reduces the air and fuel requirements for a typical Claus plant and H2S tail gas system. This process is not very popular in refineries or gas plants as it adds complexity to existing unit. This process is used for smelters where concentrated SO2 is directed to H2SO4 production as this chemical is being used in the metal manufacturing process.
The CLINTOX® and SOLINOX® process is a physical scrubber process. The completely oxidized tail gas containing only SO2 is fed to a physical scrubbing tower. The concentrated SO2 is stripped from the solvent in a second column and sent back to the Claus inlet. One advantage of CLINTOX® physical scrubbing is that whatever the feed gas SO2 concentration is, the residual SO2 in the flue is always constant because of the higher solubility of SO2 in the scrubbing solution with higher concentrations in the CLINTOX® feed gas. This self-regulation allows the Claus plant to be less sophisticated and therefore, less expensive. With such a tail gas clean-up process, sulfur recovery rates of nearly 100% are attainable with approximately 80 ppmV residual SO2 in the exhaust gas.
The LAB-SORB™, CANSOLV®, CLAUSMASTER®, CLINTOX® and SOLINOX® are all useful systems and processes useful when the target is to produce H2SO4 from SO2. However, when combined with the conventional Claus process, these processes increase the complexity of the system by requiring additional equipment and materials. In addition, the processes and systems require increases in energy use. Finally, all of these processes produce waste streams that require removal and processing.
Another type of scrubbing system is using caustic/sodium sulfite solution to capture SO2 from catalytically oxidized sulfur species. Such a system processes lean acid gas over a catalyst which oxidizes the H2S to SO2 at a temperature of about 700° F. This is desirable for low SO2 emissions as produced sodium sulfite has to be disposed in the waste water system.
Regardless of which scrubbing technology is selected, one downside of scrubbers is that they all must have a method for removing the water droplets and reaction by-products from the gas before they exit the scrubber. In addition, the processes need to provide removal of particulates in addition to acid gas removal. Most wet gas scrubbers will remove some particulates. However, another piece of equipment, such as a venturi scrubber, is often required to accomplish significant removal of particulates.
Therefore, a process which minimizes SO2 being released to atmosphere without requiring excessive amounts of energy, equipment and materials, or process shutdown is desired. Preferably, such a process, would maintain the overall sulfur capacity of the Claus unit, while increasing the overall sulfur recovery efficiency.